The third generation partnership project (3GPP) wideband CDMA (W-CDMA) system is outlined in the operational scenarios for UMTS releases R99/R4 and R5. This system utilizes TDD and FDD modes and employs multiple common and dedicated channels for establishing a communications link. The Downlink (DL) common channels include at least one primary common control physical channel (P-CCPCH) containing the BCH (broadcast channel) and/or at least one secondary common control physical channel (S-CCPCH) containing a forward access channel (FACH).
The communications link is typically established using a wireless transmit/receive unit (WTRU). A WTRU includes but is not limited to a user equipment, mobile station, fixed or mobile subscriber unit, pager, or any other type of device capable of operating in a wireless environment. These exemplary types of wireless environments include, but are not limited to, wireless local area networks and public land mobile networks. The WTRUs described herein are capable of operating in a time slotted mode or a frequency divided mode such as TDD and FDD respectively. A “base station” includes but is not limited to a Node B, site controller, access point or other interfacing device in a wireless environment.
It is known that link performance at the cell edge of a multi-cell wireless communication system has long been a concern, particularly for common channels. Link analyses have shown that a wireless WTRU on the cell edge will have block error rates (BLERs) above 10% or even higher under certain fading conditions. In addition, for optimization of capacity, it might be desirable to locate an S-CCPCH in the same slot as the P-CCPCH.
A special class of services offered by network operators and carried by S-CCPCHs will be MBMS. In wireless communication systems, MBMS are used to efficiently distribute a common data service to multiple subscribers.
MBMS differ from classic point-to-point (PtP) services such as speech or bi-directional video-conferencing, in that a group of users are intended receivers of the same message sent by the network. Realization of MBMS therefore differs from PtP services in that the latter are usually sent over user-dedicated physical channels, whereas the former are more appropriate to be sent on common physical channels to be received by multiple WTRUs. The requirements for MBMS in terms of data rate vary in the range of up to around 100 kbps, but the most common requirement indicates demand for MBMS at around 64 kbps per cell, and 90% of users in the cell covered by the MBMS.
The fundamental problem with providing MBMS in a CDMA system is that unless dedicated channels are used, it is difficult to subject the physical channel carrying MBMS to power control. Accordingly, the base station transmit power must be set such that the user of the MBMS located most distant from the base station in the serviced group can reliably receive the physical channel. In essence, the base station must support the possibility that some user in the group of N MBMS users is at the cell edge and, therefore, the transmit power is set according to that user's needs. However, for most of the users this power is far more than sufficient. This creates a disproportionate amount of interference to other users in the same and neighboring cells.
By way of example, preliminary studies for wideband W-CDMA FDD show that in order to achieve coverage of more than 90% of the WTRUs in a representative FDD cell with the 64 kbit/second MBMS, typically around 30% of the base station power would be required with MBMS sent on the physical channel. Also, it is noted that it is extremely difficult to service MBMS users at cell edge at sustainable data rates.
Therefore, there is need for reducing such a large resource demand. To this end, several schemes for reducing the required power fraction for the MBMS have been discussed to improve link performance of the MBMS channel. These include: 1) longer interleaving, i.e., longer transmission time intervals (TTIs) with better time diversity, 2) transmit diversity for the MBMS channel, and 3) improved channel coding. Using such techniques, the power fraction of an FDD base station required for supporting the example 64 kbit/sec MBMS could be reduced from 30% to around 10-20%.
For UMTS narrowband TDD (NTDD) (1.28 Mcps option), the high interference levels created by the MBMS may be mitigated in the physical channel timeslots (TS) by exploiting frequency reuse. This is possible in principle because of the smaller bandwidth per NTDD carrier. For example, three narrowband carriers can be supported within the 5 MHz spectrum allocation of FDD or wideband TDD (WTDD).
Using this scheme, some cells would transmit MBMS in a particular timeslot, TSn, on a frequency f1, a second group in TSn but on a frequency f2, and a third group in TSn but on a frequency f3. Because of the longer distance between two base stations sending MBMS in the same TS on the same frequency, more spatial separation is achieved, and therefore, less interference coming from the MBMS TS is created to other cells. However, an operator must have these three frequencies available in the deployment area. Techniques to reduce transmit (Tx) power requirements include, for example, usage of longer TTI lengths, soft handover and Tx diversity.
As a result of the previous discussions for universal terrestrial radio access (UTRA) FDD, a reduction down to some 15-20% base station DL Tx power is indicated for supporting 64 kbit/sec reference MBMS on S-CCPCH.
Previous systems disclose the implementation of an R4-based LCR TDD system in a deployment area with 3 low chip rate (LCR) carriers in a 5 MHz bandwidth and mapping the MBMS in this system onto an S-CCPCH contained in a single timeslot and assuming a frequency reuse factor 3 for this timeslot. These results show that LCR TDD can provide MBMS up to 64 kbps at a block error rate=10% (BLER=10%) or around 16-32 kbps at BLER=1% could be supported when using full base station Tx power in the S-CCPCH timeslot.
Furthermore, in a prior art communication system using a time-domain reuse factor of 3, cells in set 1 would transmit their MBMS in TSn, cells in set 2 would transmit their MBMS in TSn+1 and sets in cells in set 3 would transmit their MBMS in TSn+2. Cells in set 1 do not use TS+1 and TS+2 for any transmission, both uplink (UL) and DL, cells in set 2 do not use TSn and TSn+2 for any transmission and so on. This method works irrespective of the duration of the MBMS data block, (i.e., is independent from its TTI). The average MBMS data rate yielded per cell with this method is 170 kbps/cell and the timeslot efficiency on MBMS TSs in the system is 170 kbps/3 TSs=56 kbps/TS.
FIG. 1 shows an exemplary data frame sequence used by the above-mentioned prior art communication system, whereby a data frame is divided into TSs 1-15. The frames repeat and the TS assignments remain the same for subsequent frames until the TS is cleared or is specifically reassigned. Each timeslot is potentially assigned a predetermined number of frames.
FIG. 2 is a diagram showing channel assignments used by the above-mentioned prior art communication system. Cells in different sets are assigned different timeslots. This arrangement is used when MBMS broadcasts are transmitted from multiple sources which may have overlapping coverage areas.
To illustrate, WTRU M1 in TS1 is assigned codes corresponding to cells in a first set (set 1). WTRU M2 in TS 2 is assigned codes corresponding to cells in a second set (set 2), and WTRU M3 in TS 3 is assigned codes corresponding to cells in a third set (set 3). This appears in Frame A and repeats for subsequent frames until one or more of the assignments are changed.
Still referring to FIG. 2, in Frame A 78, the set 1 cells are assigned a first set of timeslots TS1. The remainder of the timeslots TS2-TSn is not used by set 1. The physical channel assignment for set 1 is the entire S-CCPCH. The set 2 cells are assigned a second set of timeslots TS2. The remainder of the timeslots TS1 and TS3-TSn are not used by set 2. The physical channel assignment for set 2 is the entire S-CCPCH. The set 3 cells are assigned a third set of timeslots TS3. The remainder of the timeslots TS1-TS2 and TS4-TSn are not used by set 3. The physical channel assignment for set 3 is the entire S-CCPCH. This pattern repeats for Frame B 80, with corresponding timeslots TS1-TS3 assigned to the cells in the different sets.
Note that the drawback of the above-mentioned prior art communication system is that the timeslots TS1, TS2 and TS3 cannot be used for other transmissions. Thus, if a timeslot is used for cells in one set, that timeslot may not be used for cells in another set. It would be desirable to have a set of TDD cells that are able to share a time-domain reuse pattern.
For proper application of radio resources, the Universal Terrestrial Radio Access Network (UTRAN) tracks the number of active MBMS users. Within each cell, for each MBMS the number of active users is used to determine the type of transport and physical resources applied to the MBMS, and when to initiate and terminate the MBMS in each cell.
Services are established as a result of MBMS activation and subscriber mobility. The mechanism envisioned to track MBMS users incorporates Radio Resource Control (RRC) layer 3 signaling for MBMS “joining” (service activation) and cell update procedures to maintain the subscribers location. With these tools it is possible to know which users have activated the service and in which cell the service needs to be distributed.
Due to application of closed loop power control and transmit diversity, dedicated channels are more efficient when the number of users of a particular one of MBMS is small. When the number of users increases, the dedicated channel efficiency gains do not compensate for the duplication of each data stream, and common channels that provide a single data stream to multiple subscribers are used. This method is known as transport/physical channel switching and may be applied at anytime during an MBMS transmission.
When common channels are used, it is not practical to apply ARQ techniques ensure successful delivery. Therefore each MBMS transmission is repeated to increase the probability of successful delivery. The number of retransmissions takes into account the expected BLER of the transport and physical resources applied to the service.
MBMS are transmitted several times to better ensure successful delivery. The number to retransmissions is relative to the expected channel quality. This number will take into account a worst case scenario to achieve an expected Quality of Service (QoS). One example of this is when subscribers are located at the cell edge and as a result there is a high BLER. Often subscribers will experience better radio propagation conditions and will achieve successful delivery well before the retransmissions complete.
In summary, several improvements are desired to overcome deficiencies associated with conventional MBMS are desired. Firstly, there is a need for a new scheme which supports UMTS WTDD and NTDD, and also increases the capacity of common channels for offering MBMS. Secondly, a system for improving resource efficiency using performance enhancing techniques is desired whereby a set of TDD cells are configured to share a time-domain reuse pattern. Thirdly, no explicate service delivery indication exists so that any subscriber that has activated the MBMS will be billed for reception. Therefore, it would be desired for the UTRAN to provide a sufficient number of retransmissions to ensure reliable reception.